1. Field of the Invention
The present invention relates to an electric heater, comprising a heating block which is held in a housing defining opposite frame apertures and includes parallel layers of heat-emitting and heat generating elements, and comprising a grid arrangement covering the respective frame aperture and reinforcing the housing.
2. Description of the Related Art
An auxiliary heater of this nature for conditioning the air in the interior of a motor vehicle is for example known from EP 1 564 503. The heat generating elements of the heating block normally comprise several PTC heating elements which are provided overlapping in one plane and are arranged between printed conductors which are normally formed by sheet metal bands. These printed conductors carry current with different polarities. The PTC elements can be glued to these printed conductors. It is also possible for the printed conductors to contact the PTC heating elements under tension. In any case it must be ensured that for extracting the heat produced by the PTC heating elements and for feeding current, good contact between the printed conductors and the PTC heating elements exists.
One or more heat generating elements can be provided as part of the heating block. The heat produced by the heat generating elements is dissipated through heat dissipating elements to the medium to be heated, i.e. the air. It flows through the housing through the two frame openings which accommodate the flat heating block. The frame openings here lie normally parallel to one another on oppositely situated sides of an essentially flat, frame-shaped housing. With regard to the most economical manufacture of the electrical heating device, the heat dissipating elements are generally formed from meander-type bent sheet strips, which form corrugated ribs. These corrugated ribs contact heat dissipating elements on one or both sides. Consequently, the heating block comprises several layers of heat dissipating and heat generating elements, wherein it must also be ensured with regard to the thermal emission that the heat dissipating elements have a good contact to the heat generating elements. Also in this respect, the heat dissipating elements can be permanently joined to the heat generating elements and/or contact them under tension through at least one spring element accommodated in the housing.
Instead of a meander-type sheet metal band, the heat dissipating element can also be formed by an extruded aluminium profile, which forms ridges, which extend essentially at right angles to the layers of the layer structure comprising the heat dissipating and the heat generating elements. In a case of this nature the printed conductor, i.e. the generally flat locating face, for the PTC heating element can be formed by the outer surface of an extruded aluminium profile of this nature. With both alternatives, corrugated rib element or extruded profile, the locating face for the PTC heating elements is formed electrically conducting and is electrically connected to contacts which are normally mounted insulated from one another. In the first case the contacts are generally formed by the exposed ends of the sheet metal bands.
The layered heating block consisting of parallel heat dissipating and heat generating elements, optionally with one or more additional spring elements extending parallel to it, is preferably mounted in a housing with a U-shaped cross-section. When the layer structure is subjected to the pressure of a spring, the frame has to be dimensioned such that the spring force can be continuously maintained even at increased temperatures. Here it should be noted that the insulating frame is nowadays manufactured as an injection moulded part, due partially to economical reasons. Normal housings nowadays consist of a housing lower part and a housing upper part. The housing lower part here forms a receptacle for the individual elements of the heating block and, where required, of the spring element. The individual elements of the heating block are arranged in this housing lower part. Then the heating block is enclosed in the housing by joining the housing upper part and the housing lower part. To achieve this, edges which surround the frame openings can partially cover the heating block so that the heating block is enclosed between the frame openings and mounted in the housing. The two housing parts are then joined together, for example using a latching connection.
With this type of assembly there is the problem that the individual layers of the heating block must be arranged at a predetermined point in the housing. Since not every heat generating element is assigned its own contacts, the electrical conditions within the heating block must also be considered during assembly. However to minimise the manufacturing costs, there is also the desire to form the parts of the heating block as standardised as possible, so that identical components can be used for different layers of the heating block.
Furthermore, the housing itself should be able to be manufactured as simply as possible with regard to an economical manufacture of the electrical heating device. Here however, the particular requirements for the practical installation of one or more spring elements in the housing have to be followed when on joining the housing parts the heating block is already subjected to prestressing in the frame so that joining has to take place against this prestressing.
With regard to the previously discussed problems, in EP 1 564 503 an electrical heating device of the generic type has already been suggested in which the layers of the heating block including a spring element are first mounted in a housing lower part initially free of tension. A housing upper part, which can be connected to it, forms an oblique sliding surface which extends over the end of the spring element protruding up from the housing lower part in relation to the outer side of the heating block. When joining the housing upper part and housing lower part, the spring element is accordingly compressed in the direction of the heating block and contacts it prestressed.
This prior suggestion leads to a certain simplification during assembly, which however requires that the elements of the heating block, as also the spring element, are brought into the correct positions in the housing lower part. Furthermore, the housing implemented with this electrical heating device has various oblique surfaces, which are required for stressing and enclosing the spring element when joining the housing parts.
The present invention is based on the problem to provide an electric heater, which allows a more simple and, thus, more inexpensive production. Furthermore, the present invention wishes to provide a heat generating element of an electric heater, which is suited for its installation into the aforementioned electric heater. According to another independent aspect of the present invention a housing is provided, which can be employed as a part of the electric heater according to the present invention and which is particularly suited to receive the heat generating elements proposed in accordance with the present invention.
To overcome the first-mentioned problem, the present invention proposes an electric heater comprising a planar heating block which is held in a housing defining opposite frame openings and which includes parallel layers of heat-emitting and heat-generating elements. The electric heater additionally includes a grid arrangement covering the respective frame openings and reinforcing the housing wherein the first struts of the grid arrangement extending at right angles with respect to the layers are assigned to the housing and second struts, extending parallel with respect to the layers, are defined by the heating block. This heater differs from the generic electric heater in that first struts of the grid arrangement extending at right angles with respect to the layers are assigned to the housing and in that second struts extending parallel with respect to the layers are defined by the heating block.
Contrary to EP 1 432 287 B1 the grid arrangement of the present invention is not only defined by the two housing parts connected to each other, but the housing parts merely include those struts which extend as first struts at right angles to the layers of the heating block. The grid elements extending at right angles with respect to the above and designated as second struts are defined by the heating block. The second struts thereby serve as a certain shield for the heat generating element, which comprises, as a rule, two parallel conductor paths having different polarities and PTC elements provided there between and arranged side by side. Preferably, the second strut externally overlaps the two conductor paths in the region of the frame aperture, thereby preventing that foreign particles penetrating from outside with the air to be heated through the frame opening cause a short circuit between the opposite conductor paths on the longitudinal side of the heat generating element subjected to the flow.
The first and second struts of the inventive electric heater each advantageously contribute to a certain reinforcement of the grid arrangement honeycombing the frame opening. To this end it is not necessary, however, that the struts crossing each other and provided at right angles with respect to each other be firmly connected to each other. A certain form-closure and/or a certain support of the first and second struts is rather sufficient to achieve a certain reinforcement of the grid arrangement.
The second struts are defined by the heating block, so that the position of the corresponding second struts in the housing is predefined only by the installation situation of the layers of the heating block. Thus, it is possible more easily to provide the second struts in the region of the heat generating elements, for example, in order to cover the conductor paths having different polarities on the front face. Contrary to the above-explained prior art there no longer exists the problem of a narrow tolerance adaptation of the geometric configuration of the housing, on the one hand, to the layers of the heating block, on the other hand, which are mounted herein to be sometimes movable, at least, however, with a certain play.
In this respect it is proposed in a preferred embodiment of the present invention to provide the second struts in sections between the first struts, namely in such a way that the first struts are each fixed between two sections of the first struts in a form-closed manner, which means that the first struts are only slightly movable or not movable at substantially right angles to their extension, i.e. parallel to the longitudinal extension of the second struts, and are accordingly held between the respective sections in a form-closed manner.
In the aforementioned further development the second struts can engage into recesses of the first struts, which are adapted to the dimension of the second struts, so as to produce an engagement connection fixing the two struts with respect to each other in a substantially immovable manner. With respect to the reinforcement and particularly by taking into account possible manufacturing tolerances it has proved to be advantageous, however, to arrange the second struts in the longitudinal direction of the first struts to be movable with respect to the same. This particularly means that the two struts are mounted to be slightly movable relative to each other, which allows the compensation of manufacturing tolerances and, respectively, a certain yielding of the individual layers of the heating block, for example, for compensating thermal expansions and/or as a reaction to a tensile force acting onto the heating block from outside, which is exerted by one or more springs installed in the housing.
The second struts may be formed as shielding components between the heat-emitting or heat generating elements or as part thereof. Particularly reliable is an embodiment in which the second struts are directly defined by parts of the heat generating element. It has proved to be advantageous to define the second struts by position frames made of an insulating material, which define receptacles provided side by side each for at least one PTC heating element and which are arranged between conductor paths against which the PTC heating elements are placed in an electrically conductive manner. According to this preferred embodiment the struts are formed as an element integrated in the heat generating elements, so that a correct positioning of the struts can be guaranteed in an easy manner.
To achieve a good shielding of the electric conductor paths, and in view of a desired mechanical interaction and support of the two struts, it is proposed according to a preferred further development of the present invention to place the conductor paths onto both sides of the PTC elements, i.e. to provide the conductor paths as elements which are initially separate and dissociated from the PTC elements, and to define the sections of the second struts such that they externally overlap the conductor paths, thereby captivating them to the position frame. With this preferred embodiment a preassembled unit is provided, which can be handled and installed as an integrated element during the production of the electric heater.
The constructive requirements demanded from the sections and struts, respectively, with respect to fixing the conductor paths may thereby be relatively small, if the conductor paths abut, in the assembled condition, on the PTC heating elements by means of a spring device holding the heating block in the housing under a spring pretension, which is known, for example, from EP 1 432 287 and EP 1 564 503.
In order to manufacture the second struts by taking into account a careful treatment of the material, it is proposed according to another preferred embodiment of the present invention to define the upper and lower side of the heat generating elements between the sections of the second struts by the conductor paths. The conductor paths are accordingly exposed between the sections of the second struts and are shielded and, thus, covered by the struts only after the installation into the housing. The further development moreover permits a relatively flat structure of the electric heater if relatively many standardized components are used, e.g. sheet metal bands as conductor paths.